One-Step Annulative π-Extension of Alkynes with Dibenzosiloles or Dibenzogermoles by Palladium/o-chloranil Catalysis

Article abstract of DOI:10.1002/anie.201610374

Reliable and short synthetic routes to polycyclic aromatic hydrocarbons and nanographenes are important in materials science. Herein, we report an efficient one-step annulative π-extension reaction of alkynes that provides access to diarylphenanthrenes and related nanographene precursors. In the presence of a cationic palladium/o-chloranil catalyst system and dibenzosiloles or dibenzogermoles as π-extending agents, a variety of diarylacetylenes are transformed successfully into 9,10-diarylphenanthrenes in a single step with good functional-group tolerance. Furthermore, double π-extension reactions of 1,4-bis(phenylethynyl)benzene and diphenyl-1,3-butadiyne are demonstrated, affording oligoarylene products, which show potential for application in the synthesis of larger polycyclic aromatic hydrocarbons and nanographenes.

Full text of DOI:10.1002/anie.201610374

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201610374
German Edition: DOI: 10.1002/ange.201610374
Polycyclic Aromatic Hydrocarbons
Hot Paper
One-Step Annulative p-Extension of Alkynes with Dibenzosiloles or
Dibenzogermoles by Palladium/o-chloranil Catalysis
Kyohei Ozaki, Keiichiro Murai, Wataru Matsuoka, Katsuaki Kawasumi, Hideto Ito,* and
Kenichiro Itami*
Abstract: Reliable and short synthetic routes to polycyclic
aromatic hydrocarbons and nanographenes are important in
materials science. Herein, we report an efficient one-step
annulative p-extension reaction of alkynes that provides access
to diarylphenanthrenes and related nanographene precursors.
In the presence of a cationic palladium/o-chloranil catalyst
system and dibenzosiloles or dibenzogermoles as p-extending
agents, a variety of diarylacetylenes are transformed success-
fully into 9,10-diarylphenanthrenes in a single step with good
functional-group tolerance. Furthermore, double p-extension
reactions of 1,4-bis(phenylethynyl)benzene and diphenyl-1,3-
butadiyne are demonstrated, affording oligoarylene products,
which show potential for application in the synthesis of larger
polycyclic aromatic hydrocarbons and nanographenes.
Recently, we have reported the palladium-catalyzed one-
step APEX reaction of polycyclic aromatic hydrocarbons
(PAHs) with dibenzosiloles for the synthesis of structurally
uniform nanographenes (Figure 1).[8] We have also discovered
Figure 1. Palladium-catalyzed K-region-selective annulative p-extension
(APEX) of polycyclic aromatic hydrocarbons (PAHs).
N
anographenes have recently received a considerable
amount of attention in synthetic chemistry, nanocarbon
science, and organic electronics.[1] The chemical and physical
properties of nanographenes depend on their size, shape, and
peripheral structure,[2] and, therefore, efficient and precise
synthetic routes to structurally uniform nanographenes are in
high demand. To date, cyclodehydrogenation (also known as
graphitization or graphenization) through a FeCl3-mediated
Scholl reaction, photocyclization, and oxidative coupling have
been employed frequently for the syntheses of oligo- and
polyarylene precursors for nanographenes.[3] Among the
various possible synthetic approaches to oligo- and polyar-
ylenes, the annulative p-extension (APEX) of alkynes[4–7] is
generally regarded as one of the most powerful and reliable
methods for the construction of not only small oligopheny-
lene derivatives[5–7] but also ladder polyphenylenes that are
precursors for graphene nanoribbons (GNRs).[4] Therefore,
the development of efficient one-step APEX reactions of
alkynes is an important synthetic goal in the field of nano-
carbon sciences.
that the K-region (convex armchair edge) selectivity stems
from the favorable coordination of the carbon–carbon double
bond at the K-region to a cationic palladium (K-region is the
most olefinic site within PAH structures).[8a] While the p-
coordination of PAHs (e.g., phenanthrene and pyrene) to
a metal is usually not strong, an electron-deficient and
cationic arylpalladium species (the key intermediate in our
APEX catalysis[8a]) can facilitate the p-coordination more
efficiently.
Building on these mechanistic studies, we envisaged that
this catalytic APEX reaction with dibenzosiloles could be
potentially extended to other substrates containing multiple
carbon–carbon bonds such as alkynes,[9,10] providing access to
extremely facile and short synthetic routes to oligoarylenes
and nanographenes. Among the related examples of APEX
reactions of diphenylacetylene with various p-extension units
giving 9,10-diphenylphenanthrenes,[9,10] most recently, the
group of Fukushima has elegantly demonstrated the power
of their two-step alkyne APEX reaction in the synthesis of
exotic planar/nonplanar p-conjugated molecules.[9p] Herein,
we report a novel one-step APEX reaction of diarylacety-
lenes with dibenzosiloles or dibenzogermoles catalyzed by
a palladium/o-chloranil catalyst system.
Through a screen of the reaction conditions, we were able
to establish that the alkyne APEX reaction is possible.
Treatment of diphenylacetylene (1a, 1.0 equiv) with dibenzo-
silole 2a (1.2 equiv), PdCl2 (5 mol%), AgOTf (10 mol%),
and o-chloranil (2.0 equiv) in 1,2-dichloroethane at 808C for
2 h provided 9,10-diphenylphenanthrene (3aa) in 63% yield
(Table 1, entry 1). The use of o-chloranil was found to be
crucial in this reaction. While 3,5-di-tert-butyl-o-benzoqui-
none provided the desired APEX product 3aa in 25% yield,
other common oxidants such as DDQ, p-benzoquinone, p-
[*] Dr. K. Ozaki, K. Murai, W. Matsuoka, Dr. K. Kawasumi, Dr. H. Ito,
Prof. Dr. K. Itami
Graduate School of Science, Nagoya University
Chikusa, Nagoya 464-8602 (Japan)
E-mail: ito.hideto@g.mbox.nagoya-u.ac.jp
itami@chem.nagoya-u.ac.jp
Prof. Dr. K. Itami
Institute of Transformative Bio-Molecules (WPI-ITbM)
Nagoya University, Chikusa, Nagoya 464-8601 (Japan)
and
JST-ERATO, Itami Molecular Nanocarbon Project
Nagoya University, Chikusa, Nagoya 464-8602 (Japan)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201610374.
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Table 1: Effect of varying representative reaction conditions in Pd-
catalyzed APEX reaction of diphenyleacetylene 1a with dibenzosilole 2a.
Table 2: Pd-catalyzed APEX reaction of various diarylacetylenes (1b–1j)
with dibenzosilole 2a.
Entry 2a
[equiv]
[Pd]
[Ag]
o-Chloranil Yield of
[equiv]
3aa[a]
1[b]
2[c]
3[b]
4[c]
5[b]
6[c]
1.2
PdCl2
PdCl2
PdCl2
Pd(OAc)2
Pd(CH3CN)4(SbF6)2
PdCl2
AgOTf 2.0
AgOTf 2.0
(63%)[d]
91% (94%)[d]
2%
15%
46%
2.0
1.2
2.0
2.0
2.0
–
–
–
2.0
2.0
2.0
AgOTf 1.5
87%
[a] Yield determined by 1H NMR spectroscopy using dibromomethane as
an internal standard. [b] Reaction time: 2 h. [c] Reaction time: 3 h.
[d] Yield of isolated product in parentheses.
chloranil, K2S2O8, and CuCl2 were found to be completely
ineffective (see Table S1 in the Supporting Information for
the overview of the oxidant screening results). In the next
step, the effect of changing other reaction parameters such as
the nature of the palladium catalyst and silver salt, and the
number of equivalents of dibenzosilole and o-chloranil was
investigated. Judging from the results listed in Table 1, the
most critical parameter was found to be the number of
equivalents of dibenzosilole. Namely, 2.0 equivalents of
dibenzosilole increased the yield of 3aa dramatically to
94% (entry 2). The absence of AgOTf or the use of Pd(OAc)2
catalyst in place of PdCl2/AgOTf diminished the reaction
yield noticeably (entries 3 and 4). Therefore, these results
suggest that the generation of the cationic palladium species
contributes significantly to the success of the alkyne APEX
reaction. The use of a preformed cationic complex Pd-
(CH3CN)4(SbF6)2—i.e., the optimal catalyst for APEX of
PAHs—resulted in a decrease in the yield of 3aa to 46%
(entry 5). Similarly, decreasing the amount of o-chloranil to
1.5 equivalents resulted in a somewhat lower yield of 3aa
(87%, entry 6). Thus, the optimized conditions for the APEX
of diphenylacetylene were established as: heating the mixture
of 1a (1.0 equiv) and 2a (2.0 equiv) in 1,2-dichloroethane at
808C in the presence of PdCl2 (5 mol%), AgOTf (10 mol%),
and o-chloranil (2.0 equiv).[11]
With the optimal reaction conditions successfully estab-
lished, various diarylacetylenes were subjected to the APEX
reaction with 2a (Table 2). Interestingly, chloro, bromo, and
boryl groups were well tolerated in this reaction, affording the
corresponding diarylphenanthrenes 3ba, 3ca, 3da, 3ea, and
3ia, equipped with functional groups that could be used for
further functionalization and p-extension, in high yields.
Diphenylacetylenes bearing trifluoromethyl groups (1 f and
1g) afforded 3 fa and 3ga in moderate yields. The use of 3,5-
di-tert-butyl-o-benzoquinone (o-DTBQ) instead of o-chlora-
nil in the reaction employing a substrate equipped with
a methyl ester group (1h) resulted in a higher yield (3ha, 43%
[a] 3,5-Di-tert-butyl-o-benzoquinone (o-DTBQ) (2.0 equiv) was used
instead of o-chloranil. [b] The product was obtained as a 1:1 mixture of
rotamers.
yield). The reaction of dinaphthylacetylene 1j also proceeded
well, affording the sterically congested 9,10-dinaphthylphen-
anthrene (3ja) as a 1:1 mixture of rotamers.
In the next step, a variety of p-extension units were tested
for the APEX reaction of alkyne 1a (Table 3). In the APEX
reactions examined, the dibenzosiloles equipped with elec-
tron-donating groups, 3,7-di-tert-butyldibenzosilole 2b and 2-
methyldibenzosilole 2c, were found to work effectively as p-
extending agents, affording the corresponding phenanthrenes
3ab and 3ac in an 87% and 80% yield, respectively.
Dibenzosilole 2d bearing a methoxy group was also able to
produce the corresponding APEX product 3ad in 52% yield.
In contrast to the dibenzosiloles bearing electron-donating
groups, the electron-deficient dibenzosiloles such as 3-chloro-
dibenzosilole 2e displayed a somewhat lower reactivity (3ae,
46%). In such circumstances, the use of dibenzogermole 4e,
which has a superior reactivity over dibenzosilole 2e,
furnished the same product 3ae in a much higher yield
(84%). Similarly, dibenzogermoles bearing a fluoro group (4 f
and 4g) afforded the corresponding APEX products 3af and
3ag in high yields.
Exploiting PdCl2/AgOTf/o-chloranil as the catalyst
system, we found that double APEX reactions of dialkyne
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Table 3: Pd-catalyzed APEX reaction of diphenylacetylene 1a with
various dibenzosiloles (2b–2e) or dibenzogermoles (4e–4g).[a]
Figure 2. Double APEX reaction of diynes. a) Reaction of diyne 1k:
Conditions i) 1k (0.20 mmol), 2a (8.0 equiv), o-chloranil (6.0 equiv),
PdCl2 (10 mol%), AgOTf (20 mol%), ClCH2CH2Cl (2.0 mL), 808C,
18 h. b) Reaction of diyne 1l: Conditions ii) 1l (0.20 mmol), 2a
(8.0 equiv), o-chloranil (6.0 equiv), PdCl2 (10 mol%), AgOTf
(20 mol%), ClCH2CH2Cl (2.0 mL), 808C, 16.5 h. c) Thermal ellipsoid
representations (50%) of the X-ray crystal structures of 3ka and 3la,
as obtained with ORTEP.[12] Hydrogen atoms and solvent are omitted
for clarity.
[a] Reaction conditions: diphenylacetylene (1a) (0.20 mmol), dibenzo-
silole 2 (3.0 equiv) or dibenzogermole 4 (2.0 equiv), PdCl2 (5 mol%),
AgOTf (10 mol%), o-chloranil (2.0 equiv), 1,2-dichloroethane (2.0 mL),
808C.
substrates were also possible. For example, 1,4-bis(phenyl-
ethynyl)benzene (1k) and 1,4-diphenyl-1,3-butadiyne (1l)
underwent double APEX reaction with dibenzosilole 2a
(Figure 2a,b). In these reactions, the addition of larger
quantities of dibenzosilole 2a and o-chloranil was necessary
in order to produce the double APEX products (3ka and 3la)
in reasonable yields. In particular, these additional reagents
were necessary because the dibenzosilole and o-chloranil
were being consumed through homo-dimerization, desilyla-
tion, and decomposition faster than in the APEX reactions of
1k and 1l. The sterically congested structures of 3ka and 3la
were confirmed by X-ray crystallographic analysis (Fig-
ure 2c).[12]
Two possible reaction mechanisms for the APEX reaction
of alkynes are shown in Figure 3. Based on our previous
mechanistic studies of APEX reactions of PAHs,[8a] the
reaction is most likely to be initiated by a transmetalation
of dibenzosilole 2a with a cationic palladium(II) to give
a biphenylpalladium intermediate A. Subsequent p-coordi-
nation of alkyne 1a and carbopalladation afford alkenylpal-
ladium intermediate B, which undergoes intramolecular
Figure 3. Proposed mechanisms for the Pd-catalyzed APEX reaction of
diphenylacetylene (1a) with dibenzosilole 2a.
transmetalation to generate palladacycle D. Alternatively,
palladacycle D might be formed through a transmetalation–
carbonpalladation sequence from A (via C). The product of
the APEX reaction, 3aa, would be obtained by the reductive
elimination from intermediate palladacycle D along with Pd0
species, which can be oxidized by o-chloranil to regenerate
the PdII species.
In summary, palladium-catalyzed APEX reactions of
alkynes with dibenzosiloles or dibenzogermoles have been
demonstrated. The PdCl2/AgOTf/o-chloranil catalyst system
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Acknowledgements
This work was supported by the ERATO program of the JST
(K.I.) and JSPS KAKENHI Grant Numbers JP26810057 and
JP16H00907 (H.I.). We thank Dr. Yasutomo Segawa and
Takao Fujikawa for X-ray diffraction analyses. We thank Shin
Suzuki for providing some diarylacetylenes. ITbM is sup-
ported by the World Premier International Research Center
(WPI) Initiative (Japan).
Conflict of interest
The authors declare no conflict of interest.
Keywords: alkynes · dibenzosiloles · palladium ·
polycyclic aromatic hydrocarbons · p-extension
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Manuscript received: October 23, 2016
Revised: November 24, 2016
Final Article published: && &&, &&&&
4
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Communications
Polycyclic Aromatic Hydrocarbons
K. Ozaki, K. Murai, W. Matsuoka,
K. Kawasumi, H. Ito,*
K. Itami*
&&&& — &&&&
One-Step Annulative p-Extension of
Alkynes with Dibenzosiloles or
Dibenzogermoles by Palladium/o-
chloranil Catalysis
Extending circumstances: An efficient
one-step annulative p-extension reaction
of alkynes provides facile access to
diarylphenanthrenes and related nano-
graphene precursors. In the presence of
a cationic palladium/o-chloranil catalyst
system, a variety of diarylacetylenes are
transformed through the use of dibenzo-
siloles or dibenzogermoles as p-extend-
ing agents into 9,10-diarylphenanthrenes
with good functional-group tolerance.
Angew. Chem. Int. Ed. 2016, 55, 1 – 5
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!